This invention relates to molecular sieves and processes for their manufacture. More especially it relates to a variation in the preparation of synthesis mixtures to control product characteristics. The invention relates primarily to the manufacture of phosphorus-containing molecular sieves, especially silicoaluminophosphates.
In U.S. Pat. No. 4,440,871, the preparation of a number of phosphorus-containing molecular sieves is described.
The patent particularly describes processes for the manufacture of numerous crystalline microporous silicoaluminophosphates (SAPO""s) including SAPO-34, employing sources of silicon (e.g., a silica sol), aluminium (e.g., hydrated aluminium oxide), and phosphorus (e.g., orthophosphoric acid), and an organic template, for example tetraethylammonium hydroxide (TEAOH), isopropylamine (iPrNH2) or di-n-propylamine (DPA). The patent, the disclosure of which is incorporated by reference herein, gives X-ray diffraction data for the SAPO""s and describes their utilities in catalysis and absorption.
International Application WO 00/06493 describes obtaining phosphorus-containing molecular sieves of lower particle size and narrower size distribution by agitation, e.g., stirring or tumbling.
EP-A-541 915 is concerned with the conversion of methanol to olefins (MTO), especially light (C2 to C4) olefins using an aluminophosphate crystalline molecular sieve catalyst. The specification describes the advantages of small particle size catalysts in MTO processes, and provides a process for facilitating the manufacture of a small particle size material by stirring the synthesis mixture, producing SAPO-34 of median particle diameters, expressed as a mass distribution, in the range of about 0.6 to 1.4 xcexcm.
EP-A-185 525 describes a process in which SAPO-37 is manufactured using a two-phase synthesis mixture. In an example there are used an aqueous phase containing phosphoric acid, alumina, and tetraethyl and tetrapropyl ammonium hydroxides as organic templates, and an organic phase comprising tetraethyl orthosilicate in hexanol, a solvent immiscible with water. In a comparative example, the silicon source, silica (Hisil), is dispersed in the tetrapropylammonium hydroxide.
International Application WO 01/36328 describes a process in which a metalloaluminophosphate molecular sieve is manufactured using an aqueous synthesis mixture comprising a template, sources of the elements essential to the structure of the sieve and an organic solvent miscible with water, the purpose of the solvent being to solubilize the source of the metal in the aqueous synthesis mixture. When the metal is silicon, the source may be a tetraalkyl orthosilicate.
The present invention is based on the observation that if the silicon-providing component is dissolved in an organic base prior to its incorporation in the synthesis mixture the resulting silicoaluminophosphate molecular sieve has a particle size smaller than that obtained from an otherwise identical synthesis procedure in which the silicon-providing component is added directly or is merely dispersed in another component of the synthesis mixture.
The present invention accordingly provides a process for the manufacture of a silicoaluminophosphate crystalline molecular sieve which comprises providing sources of aluminium, of phosphorus and of silicon, the source of silicon being a solution of the silicon component in a water-miscible liquid organic base or in an aqueous solution of a solid organic base, forming a synthesis mixture from said sources and any other material necessary to form the molecular sieve, and treating the synthesis mixture for a period and at a temperature appropriate for the manufacture of the molecular sieve.